For example, in-vehicle sensors, including a collision avoidance radar, a sensor for measuring the number of rotations of wheels, and a sensor for detecting the rudder angle of a steering wheel, have heretofore been networked over, for example, a controller area network (CAN) bus.
In such a network over which multiple sensors are connected to the bus, the sensors must be synchronized with each other in order to improve the transmission efficiency of the bus and to cause the sensors to perform the measurement in accordance with a common time base.
Networks in which sensors connected to a bus operate in synchronization with each other are disclosed in Patent Documents 1 to 3.
Patent Document 1 discloses a network in which an uppermost-stream data transmission device generates a synchronization signal indicating a timing of data acquisition and each downstream data transmission device samples a signal from a current sensor on the basis of a sampling signal generated by correcting the synchronization signal based on a propagation delay time.
Patent Document 2 discloses a network including multiple computer terminals and a time-management computer terminal (reference terminal), which measures a reference time for all the computer terminals connected over the network. In this network, when a measurement computer receives a processing request from the time management computer terminal, the measurement computer records the measurement data output from the sensors in the measurement computer, along with time information that is received.
Patent Document 3 discloses a system in which a sensor connected to a network follows a measurement sequence in accordance with a trigger signal generated by another sensor to perform measurement and calculation. In this system, the sensor is capable of performing a predetermined measurement operation in synchronization with the trigger signal supplied from the other sensor.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-94779
Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-97506
Patent Document 3: Japanese Unexamined Patent Application Publication No. 10-111151
However, the network disclosed in Patent Document 1 can be used only under the condition that a constant propagation delay time is always ensured. In other words, the network cannot be used if the timing when the data is output to the network can be varied due to, for example, bus arbitration.
Since the reference clock (the time management computer terminal) for the synchronization is required in the network disclosed in Patent Document 2, the network becomes complicated and is increased in cost.
Since the device for generating the trigger signal is required for every sensor in the network disclosed in Patent Document 3, the system becomes complicated and is increased in cost. In addition, since each sensor operates on the basis of the trigger signal generated by another sensor, the measurement and calculation times of a certain sensor cannot be determined. Accordingly, after one sensor completes the measurement and calculation, another sensor operates. As a result, there is a problem in that it is not possible to concurrently operate multiple sensors and, therefore, a higher usability of the network cannot be achieved.